1. Field of the Invention
This invention relates generally to a digital signal transmitting and receiving system and, more specifically, to a system for use in transmitting a digital signal such as might be derived by digitally converting an analog stereo signal, announcement signal, facsimile data signal, or a computer game program, on a cable television transmission line using a single, unused television channel having a bandwidth of approximately 6 MHz.
2. Description of the Background
In order to better understand the signal transmitting system of this invention an overall system suitable for employing the present invention is first described. The present invention is intended to employ a cable television (CATV) transmission line and FIG. 1 shows a digital signal transmitting system for use with such transmission line. Input terminals 201-204 are arranged to receive analog signals, which might be analog stereo signals, and such input signals are fed to analog-to-digital convertors 207-210, respectively. The resultant digital signals from analog-to-digital convertors 207-210 are then fed to a multiplexer 213 that produces at its outputs two serial data streams that represent the four input signals having been time division multiplexed. Time division multiplexing is a known approach to transmitting a number of signals over a commmon path by using different time intervals for the transmission of the intelligence of each message signal. Then, the time division multiplexed digital signals from multiplexer 213 are fed to a filter 214, which is provided to suppress intersymbol interference that causes code error. Filter 214 may advantageously comprise a binary transversal filter having tap coefficients adjusted so that the modulation signal satisfies Nyquists's first criterion. The output of filter 214 is fed to a four-level convertor 215 which may be thought of as operating as a digital-to-analog convertor so that it converts the input digital signals to a four-level, base-band signal. This four-level analog signal produced by four-level convertor 215 has four different amplitude values ranging from zero to three, which are respectively expressed as "0"+"0"="0", "0"+1"="1", "1"+"0"="2", and "1"+"1"="3". This pseudo-analog output signal is fed to an amplitude-modulation (AM) modulator 216 wherein it AM modulates an intermediate frequency signal (IF) having a frequency of 38.9 MHz, for example, supplied by oscillator 217. The output of the AM modulator 216 is fed through a vestigial side-band filter 218 to a mixer 219. This vestigial side-band modulation is the same as in conventional television transmissions. Thus, the filtered signal is mixed in mixer 219 with an RF signal (f.sub.c +f.sub.if) supplied by a local oscillator 220. The output signal of mixer 219 represents a modulated signal having a carrier frequency f.sub.c of 97.25 MHz, for example, and such output signal is fed through a bandpass filter 221 to output terminal 222 as the modulated, system output signal, the bandwidth of which is limited to 6 MHz. The output signal developed at output terminal 222 is then fed to a head end of a CATV system (not shown). Thus, the original input signals are placed in an unused television channel on a conventional cable television transmission line and require no more bandwidth (6 MHz) then a typical single television channel.
FIG. 2 shows a system for "receiving" a signal as might be placed on the CATV transmission line by the sytem of FIG. 1, in which the modulated signal transmitted through the transmission line of the CATV system is supplied at input terminal 231 to a wide bandwidth receiver front end 232 where it is amplified and converted to an intermediate frequency (IF) signal of 58.7 MHz, for example, and this intermediate frequency signal is supplied to a phase-locked loop (PLL) synchronous detector 233, which functions as an AM detector, so that the four-level, base-band signal, as produced by the four-level convertor 215 of FIG. 1, is demodulated. An automatic gain control (AGC) circuit 246 is provided with an input from PLL detector 233 and produces an output control signal fed to front end 232 to prevent overloading of the front end amplifier. The output signal from the phase-locked loop detector 233 is fed to a level comparator 234, which operates as a kind of analog-to-digital convertor, by demodulating the detected signal and producing a series digital signal having four possible values, "0", "1", "2", and " 3", on the basis of whether the output signal from the PLL detector 233 exceeds a reference level, as represented by a so-called eye pattern. The eye pattern is generally known in data transmission by an oscilliscope display of the detector voltage waveform in a data modulator/demodulator. This pattern gives a convenient representation of cross-over distortion and can be derived in the known fashion, based upon the overall frequency characteristics of the system and the relationship thereto between the Nyquist frequency and the transmission capacity of the system in bits per second.
The digital signal thus essentially demodulated by level comparator 234 is fed to a demultiplexer 235. Also produced by the level comparator 234 is a bilevel synchronizing signal fed to a clock reproducing circuit 245 that produces a bit clock signal applied to demultiplexer 235 to control the output thereof in the appropriate time-division manner. Also produced by clock reproducing circuit 245 is a synchronizing signal fed to both the automatic gain control circuit 246, as well as to demultiplexer 235. Demultiplexer 235 then produces a plurality of digital signals in a time-division manner that are supplied, respectively, to digital-to-analog (D/A) convertors 239-242, so that analog signals corresponding to the original input signals as applied at inputs 201-204, (FIG. 1) are respectively developed at output terminals 243-246.
Although the system described hereinabove would appear to be a workable and feasible system, in actuality such system is not available for use in an effective and usable form, principally because of the lack of an economical method of addressing the receiver terminals.
The problem is that in a transmission system, such as cable television, in which the signal is transmitted through a cable, that is, hard wired, at the receiving side at which the signals are being distributed there may be fewer than several hundred receivers, or there may be up to several tens of thousands of receivers. In other words, there is a wide spread in the number of receiving units that may be connected to the cable television transmission line. In the proposed systems, each of the receiver terminals usually has an individual address signal and the transmitter side then transmits a control signal corresponding to each address number, wherein the receiving state of each receiver terminal can be controlled.
In the systems proposed to use cable television tranmission lines as outlined above, the control signal that corresponds to each address is formed as a bit series, which can cover the maximum number of receiver terminals and in most cases this involves a bit series of approximately 20 bits. The control signal used to perform such addressing is then transmitted using a special address network line that is different from the data network line. Accordingly, if such address network line is designed to permit it to accommodate a system having a large number of receiver terminals, for example, ranging from several tens of thousands to several hundred thousand, then this address network line will be uneconomical and will be too sophisticated and expensive for systems having substantially fewer terminal receivers. On the other hand, if the address network line is designed to accommodate a system having, for example, fewer than several hundred terminals then such address network line is almost unusable when applied to a system having a substantially greater number of receiver terminals and, thus, the address network lines must be increased correspondingly.
Accordingly, as described above, while the concept of a system for transmitting digital signals on a cable television transmission line is feasible, the data addressing system is not available such that the address network lines need not be changed in accordance with the scale of the system, in order to eliminate the redundancy and uneconomical provision of more address network lines than required for the smaller size system. Furthermore, because special address network lines must be provided, the data addressing system becomes complex in its circuit arrangement and is thereby expensive in view of the associated manufacturing costs.